1. Field of the Invention
The invention relates to improvements and modifications of an osmotically driven fluid dispenser.
2. Description of the Prior Art
The invention is an improvement in the osmotically driven dispensers described in commonly owned U.S. Pat. No. 3,760,984 and Canadian Pat. No. 949,513 (corresponds to U.S. Ser. No. 291,686 filed 25 Sept. 1972). Therefore these patents are believed to be relevant prior art, with the dispenser referred to in Canadian Pat. No. 949,513 as the "minipump" being especially relevant. The minipump is an osmotically driven dispenser whose size makes it especially useful as a therapeutic system for administering drugs to animals and humans. Its basic components are an inner flexible bag that holds the drug charge, an intermediate layer of an osmotically effective solute composition, such as an inorganic salt, that encapsulates the bag, an outer shape-retaining membrane that is permeable to water and that encapsulates both the layer of osmotically effective solute composition and the bag, and a filling/discharge port that communicates with the interior of the bag.
In operation the bag is filled with drug via the filling/discharge port and placed in an aqueous environment, such as a body cavity or within body tissue. Water is imbibed from the environment by the osmotically effective solute through the membrane into the space between the inner flexible bag and the membrane. Since the bag is flexible and the membrane is rigid, the imbided water squeezes the bag inwardly thereby displacing drug out the filling/discharge port.
Minipumps of the above described structure and operation perform well but are not beyond being improved. Two shortcomings of those dispensers are, (1) they tend to lose drug in bulk through the filling/discharge port, especially when the dispenser is placed in an environment that experiences significant movement, and (2) air bubbles trapped in the bag during filling tend to completely or partially plug the filling/discharge port. The former shortcoming affects the constancy of the dosage rate adversely and may cause overdosing. The latter shortcoming may temporarily stop the dispensing or cause the dosage rate to be erratic or unpredictable. The present invention is directed towards eliminating or reducing both of these shortcomings.